Along with the information explosion accompanying the development of computer networks, in particular, expansion of applications for large-scale data centers to meet the needs of cloud computing, there have been strong demands for higher capacity and power saving of memory and storage.
Spintronic technology has been under research and development as the next-generation technology that can achieve both high-capacity recording and power saving operation. At the core of the technology are magnetic recording devices, such as hard disks or magnetic memory devices (MRAMs), that use reversal of magnetic poles (N/S) for recording. In such technology, miniaturization and higher capacity have been achieved by techniques which are extensions of a technique of manipulating magnetic poles by externally applying a magnetic field. However, these techniques are approaching a technological limit, and there have been demands for establishing a manipulation technique based on a new principle. An electric field-induced magnetization switching technique in which magnetic poles are reversed by switching an electric field is anticipated for application as an ultimate energy-saving manipulation technique in which joule losses can be substantially ignored.
In recent years, a magnetoelectric effect (hereinafter, referred to as the “ME effect”), which is a cross correlation of electricity and magnetism, has been receiving attention as a technique capable of controlling magnetism by an electric field. Non-Patent Document 1 (H. Xe et al., Nature Materials, 9, 579(2010)) discloses a technique in which by applying an electric field and a magnetic field in a superimposed manner, exchange coupling generated at the joint interface between a ferromagnetic material and an antiferromagnetic material having an ME effect is modulated, and thus magnetic properties of the ferromagnetic material are manipulated. Xe et al. shows that magnetic properties of the ferromagnetic material can be changed only by switching the applying direction of an electric field. Patent Document 1 (U.S. Pat. No. 7,719,883) discloses a device, such as a so-called MRAM, in which two different resistive states are achieved in a magnetoresistive effect element, such as a spin valve or a TMR element, using the ME effect. Patent Document 2 (U.S. Pat. No. 8,724,434) discloses a technique of an ME recording system in which the ME effect is used for high-capacity magnetic recording, such as that in a hard disk.
In order to apply the ME effect in a hard disk or a magnetic recording device, such as an MRAM, it is required to achieve a state in which magnetic poles, i.e., the N pole and the S pole, are fully reversed. For this purpose, it is required to set the magnitude of exchange coupling generated at the interface (which is actually observed as exchange bias (HEX)) to be larger than the value of coercive force (HC) of the ferromagnetic material. On the other hand, when the value of exchange coupling generated at the interface is excessively large, the amount of energy required for the ME effect increases excessively, which is a problem. In order to use the ME effect for magnetic recording, a technique is required in which the magnitude of coercive force and the magnitude of exchange coupling are appropriately adjusted.
Non-Patent Document 1 and Non-Patent Document 2 (P. Borisov et al., Phys. Rev. Lett. 94, 117203(2005)) disclose a structure in which the magnitude of exchange coupling can be adjusted. In these documents, the authors disclose a structure in which Pt, Pd, or the like is inserted at the interface between an antiferromagnetic layer having the ME effect and a ferromagnetic material layer. However, in this structure, in the temperature range in which ME manipulation is possible, the magnitude of coercive force is larger than the magnitude of exchange coupling. As a result, although the exchange coupling can be changed, the magnetic poles of the ferromagnetic layer are not reversed.